In vehicles, an electric generator and a secondary battery are provided for supplying power to various electric loads. In response to great demand for improvement in fuel consumption in recent years, a lithium ion battery and the like having a high energy density are being employed for second batteries, as power extension measures for conventional lead batteries. In order to safely use such a battery, it is necessary to detect charge/discharge current, battery voltage, and the battery temperature of the battery, and in addition, the state of charge (hereinafter, referred to as SOC) of the battery. In calculation of the SOC, a current sensor (hereinafter, referred to as a charge/discharge current sensor) for detecting charge/discharge current of the battery serves an important role.
Therefore, if the charge/discharge current sensor is abnormal, it is necessary to immediately detect failure and take a fail-safe measure which enables evacuation travelling while protecting the battery. Accordingly, the following method has been proposed: when failure of the charge/discharge current sensor is detected, SOC estimation for the battery is continued using total current outputted from current sensors (hereinafter, referred to as electric load current sensors) provided for respective electric loads of the battery (see Patent Literature 1).